In today's power transmission and distribution systems, reactors are used to introduce an inductive reactance into the corresponding electrical circuit. A reactor can also be called an inductor. Its main component is a coil of insulated wire which can either be wrapped around a core of magnetic material, i.e. an iron core, or can be constructed in the form of a hollow body, i.e. a hollow cylinder or a hollow cuboid, with no magnetic material inside. The latter group of reactors is known as air-core reactors.
Reactors are used in power systems for example as filter reactors to filter out undesired harmonics in a current transmitted to a power network, as shunt reactors to compensate for capacitive reactive power, as neutral-grounding reactors to limit the line-to-ground current of a directly earthed network or as current-limiting reactors to limit short-circuit currents.
The winding of a reactor used under high-voltage and high-current conditions of a power system produces considerable heat. Therefore, appropriate cooling is necessary to reduce the temperature in the reactor coil in order to minimize the losses and to avoid thermal ageing of the insulating material. The cooling of an air-core reactor can be provided by insulating the reactor coil in a cooling fluid or by letting air flow alongside the coil windings. Air-cooled reactors are also known as dry-type reactors.
In high voltage direct current (HVDC) power transmission systems, power is transmitted between two AC power networks which are connected via a DC link. Accordingly, an AC/DC and a DC/AC converter are installed at one side of the DC link, respectively. The converters can be either of line commutated converter type or of voltage source converter type. In case of a line commutated converter, a reactor is used to remove current ripples on the DC side of the converter. This reactor is called a smoothing reactor. When voltage source converters are used in the HVDC system, additionally a reactor called converter reactor or phase reactor is used on the AC side of the converter to mainly block harmonic currents arising from the switching of the converter. Apart from blocking harmonic currents, the converter reactor serves the additional purposes of providing active and reactive power control and limiting short-circuit currents. Both reactor types and their arrangement in an HVDC system are for example known from the brochure “It's time to connect”, issued by ABB Power Technologies AB, Grid Systems-HVDC, SE-771 80 Ludvika, Sweden, www.abb.com/hvdc.